You're assuming the port isn't a restriction, and it also depends a lot on your throat size and valve seat.

NONETHELESS, L/D=.25 is in general a good target to hit with a good head on a hot street engine.

Not all bbc heads are bad flowing heads. Bbc was made years after the small block and chevy addressed a lot of the flow issues the small block had with the larger boore big blocks.

I agree its not worth the money or time to port small block heads. Aftermarket heads are so cheap. But maybe not bad idea with bbc heads since there are no real cheap options. Might be worth the cash to get them cut by a pro. Depends if you got someone that can port at a good price.

Larger ports do not grant blessed more air/fuel into the cylinder. Most of the time the opposite is true.

Throat size? Throttle plate? Rare to get turbulence in the carb do to high flow rate. Usally that can be seen with a vacum guage at wot. Single and dual plane intakes really remove most of the issues that could result in engines that have very long tuned single runner intakes. In the big chevy were only talking about single runner for a few inches. Well past the carb.

Valve seat? You mean valve size as measured at the seat? Or the seat insert itself. It should be cut back if bored to larger valve if it came from the factory with larger valves then it is already taken care of. If you talking about bad machine work. I would hope that is not going to be an issue.

The reason you rasie the valve to the 25% of lift in most cases the valve is considered to be out of the head. Or the head will flow the same as if the valve was removed from the head minus stem diameter. This is always a goal in any engine from an engine with 3000rpm tq peak up to 7000 rpm peak. You will get your best performance from the engine with proper lift ratio. Now with small chevy it is not always needed because the engine make the same power with a few more points of duration but in big block it plays a larger effect.